1. Field of the Invention
The present invention relates to a processor such as a DSP and a switching power supply apparatus including the processor.
2. Description of the Related Art
In general, a conventional processor such as a microprocessor or a digital signal processor (DSP) includes blocks such as an input/output unit (for example, an A/D converter or a D/A converter), registers, a memory, and an arithmetic and logic unit (ALU), and an analog input signal is processed after being converted to digital data by an A/D converter. The processing speeds of processors basically depend on the clock frequencies thereof for the same bus and the number of operation bits. Basically, the higher the clock frequency of a processor, the higher the processing speed and cost of the processor. For example, a DSP including an A/D converter is disclosed in TEXAS INSTRUMENTS Technical Document “TMS320x280x2801x, 2804x Analog-to-Digital Converter (ADC) Module Reference Guide”.
FIG. 1 is a block diagram of an A/D converter illustrated in the Texas Instruments Technical Document.
Referring to FIG. 1, analog multiplexers 50A and 50B respectively select analog input ports each having 8 channels (totally 16 channels) in accordance with selection signals provided from sequencers 56A and 56B described later. Sample/hold circuits 51A and 51B sample and hold analog signals selected by the analog multiplexers 50A and 50B. An A/D converter 52 converts a sampled voltage of either the sample/hold circuit 51A or 51B to digital data under the control of a sequencer arbiter 57. A multiplexer 53 outputs the data to either a result multiplexer 54A or 54B selected by the sequencer arbiter 57. The result multiplexers 54A and 54B store the A/D conversion result in a memory 55 in accordance with a select signal provided from the sequencers 56A and 56B. A predetermined one of the sequencers 56A and 56B performs selection from among the analog multiplexers 50A and 50B and from among the result multiplexers 54A and 54B in accordance with an externally provided trigger, and receives an instruction signal and a termination signal of an A/D conversion start signal for the sequencer arbiter 57.
In this manner, a voltage signal of a specified analog input port is A/D converted and stored in the predetermined memory. A CPU within the DSP performs predetermined processing in accordance with the value in the memory 55 (on the basis of the A/D converted result).
Such a DSP described above may be used in the area of switching power supply apparatuses. A typical switching power supply apparatus employs a configuration in which an output voltage is stabilized by monitoring an output voltage, comparing the output voltage with a reference voltage, and performing negative feedback control of switching control in accordance with the comparison result. In this case, on the basis of the value stored in the memory 55 illustrated in FIG. 1, the CPU within the DSP performs control so as to monitor a current flowing through an inductor or the primary winding of a transformer and, when the current value becomes zero, changing the value of an output port to which a circuit for generating a switching control signal is connected, thereby turning on a switching device.
Other than such negative feedback control of an average value as described above, a current mode method is known in which a current flowing through a primary side inductor or the primary coil of a transformer is monitored and a switching device is turned off when the peak value or the one-cycle average value of the current reaches a certain threshold.
The current mode driving method has an advantage of high responsiveness to variation in load or input voltage. Among the current modes, a mode which requires particularly high responsiveness is a driving mode called a “critical mode”. In this critical mode, a current flowing through an inductor or the primary coil of a transformer is monitored and upon detection of an instant at which the current becomes zero, a switching device is turned on.
A current-resonance switching power supply apparatus monitors a current flowing through an inductor and detects an instant at which the current becomes zero and thereby turns on a switching device in order to realize zero current switching (ZCS). This control has an advantage in that switching loss is minimized.
However, there has been a problem in that a high-speed and high-cost analog comparator is needed in the case of using analog control for the current-mode or current-resonance switching control. This is because, when there is a time lag in the operation of the comparator, the current mode control, for example, loses its original advantage of high responsiveness, and a current resonance switching power supply loses its original advantage of low switching loss.
Also in the case of using digital control, the following operations are performed: analog data such as an output voltage or an inductor current is first converted to a digital value using an A/D converter, the digital value is stored in a memory and compared with a reference value by a CPU, and the duty ratio of a PWM pulse is computed in accordance with the comparison result. Since the inductor current value may possibly be an abnormal value including noise in single-point detection, a plurality of points are generally sampled during a cycle of switching and the average is computed. This naturally leads to an increased amount of computation. Since the CPU is in charge of performing all other operations in each cycle while keeping timings in synchronization with a clock frequency, a high-frequency, i.e., high-cost processor, is naturally required if the responsiveness at the time of the inductor current becoming zero is to be increased.